Exceedig maximum 65psi
 

I have a 700 x 32 tire, maximum pressure is 65 psi. How much can I SAFELY exceed the 65psi limit? I figure a harder tire means less rolling resistance. Also at 65psi the ride is too cushioned.

The max psi rating on tires is usually very (too) conservative. 80 psi should be no problem, even 100 psi.

I've never lost any sleep in overinflating a tire on a hook bead rim. I run my 700 x 32c tires a bit over the recommended limit (around 5 psi). When exceeding the recommended limit, I pump the tire up the night before and see what happens. I figure if the tire doesn't blow off the rim, I'm good to go. In any case, I wouldn't go more than 10 psi over the recommended limit. YMMV obviously and blowing a tire off the rim is no fun.

There is a balance between too much and too little air. Too much air does not improve rolling resistance, just a harsher ride. I like my tires to squish a little when I sit on the bike. Maybe you should try 28's? I'm at 235 lbs and run 50 psi front, 55 rear with 700 x 35's

Higher pressures doesn't mean lower rolling resistance. The sidewall pressure is 1/2 the pressure that will blow the tire off of the rim. Use this chart: http://www.bikequarterly.com/images/BQTireDrop.pdf

What tire are you running?

Where did you get that 1/2 blow off pressure number? I think that it's an "old bike riders fable". Maybe whoever started it is dead now.

1/2 of the tire/rim interface is the rim. For that number to be true there would have to be an industry document somewhere that specified a standard rim for testing tire blow off pressure. I've seen that 1/2 blow off pressure thing bandied about a lot but nobody has ever been able to answer my question about it's origin.

Let's do some math.

Surface area of a torus is 4 x 3.14 x 3.14 x R x r.

Using 15" for R and .75 for r:

4 x 3.14 x 3.14 x 15 x .75 = 444 square inches.

At 100 psi, that's a total of 44,400 pounds of pressure. High pressures in large tires mean literally thousands of pounds of force trying to tear your rim apart.

And as others have already said, harder tires do not roll better.

I have had good luck commuting with the panaracer pasella tourguard.

I once got a couple cheap 26x1.5" tires that specified 40psi max. Used them always at 50 till they wore out, never a problem.

Don't you find that pressure extremely mushy and slow? Your tires are rated at max 90 PSI. I tend to fun my inflation levels on the soft/comfort side. I run Schwalbe Kojaks 700 x 35. They are rated at max of 95 PSI and I run 80 and weigh less than you. You aren't over working your engine a bit?

Back when I started with a department store 10-speed, I did the pressure hero thing, using 90 psi in tires marked 65 psi. One day I parked it in the bright sun and when I came back, both tires had blown off the rim and big pieces of shredded inner tube littered the sidewalk. Wonder if it scared the bejeezus out of a passer-by. Fun times.

I favor comfort and traction. Ever ride in MA? potholes etc. I also commute on dirt paths and singletrack sometimes. I ain't racing. I've run higher and lower, this is what works for me & my conditions. When I want to over work the engine , I mountain bike in 90F heat or 6" of snow.

Second hand info from a local bike dealer. He was told this tidbit from one of his product salesman. The manufacturer can't know the vehicle weight of the bike and rider so uses a number that will cover his ass.

I see. Very good, carry on.

In other words it's a second or third hand assumption from somebody who was trying to sell him tires.

Rolling resistance is largely a function of sidewall deflection. At any given pressure, a wider tire will have less sidewall deflection than a narrow tire. Narrow tires achieve low rolling resistance by using high pressures, at the expense of rider comfort. Narrow tires are arguably more aerodynamic than wide tires, but at the speeds most of us ride it's not a significant difference.

I understand all that, hence my question. A 235 lb rider on 700 x 35 at 55 PSI = mucho deformation.

Nope, the coveted 15%. Do you mt bike at all ? I'm riding dirt, bad trails w/ potholes, double track, and single track on a cross check, YRMV.

See my previous post.

That's great. I originally asked the question only because based on my own personal experience, and charts, 55psi would result in far greater deformation. You are achieving a small amount and it works well for your riding conditions, and that's all that matters.

If you read the article tests have shown that narrow tires have a higher rolling resistance.

What article are you referring to? This one, linked earlier:

http://www.bikequarterly.com/images/BQTireDrop.pdf

Is just about what pressure gives a 15% drop for a specific weight.

Read the entire article!

If you favor a more jarring less cushioned ride then go with a narrower tire. However if you are looking to reduce pedaling effort then a better tire with more flexible sidewalls and less tread should be considered.

Well of course, overinflation should depend on the quality of the tires/tubes. If you have good tubes ( fairly new, not overly patched ), and quality tires that are in good condition ( sidewalls not weather checked and cracking ), overinflating them from 10% to 15% should be fine. I usually run Continential tubes and tires, size 700x32c, rated at 70 PSI. I usually run the back tire at 82 to 84 PSI and the front tire at 80 PSI. I'm 5' 11" and weigh in at 200 pounds. usually the air pressure indications on the side of a bicycle tire are there to "protect" the tire manufacturer against lawsuits. High pressure tires usually tend to lose air pressure over time. The narrow road tires of 110 to 120 PSI tires will lose more air pressure more quickly than the wider sized lower pressure tires you are discussing. My tubes/tires will hold near the 80+ PSI pressure for about a week before dropping down towards the recommended maximum of 70 PSI. I'll usually air them up where I want every 10 days or so. You're certainly right about the higher pressures have less rolling resistance.